The present invention relates generally to liquid crystal display devices and to methods of driving such devices. More particularly, but not exclusively, this invention relates to techniques that are effective for use with drive methods for inverting the polarity of gradation voltages that are applied to picture elements or “pixels” in groups of a plurality of lines as a unit, such as N-line inversion drive methods.
Liquid crystal display devices of the active matrix type, having switching-driven active elements (e.g. thin-film transistors) for each pixel, are widely used as display devices for use in personal computers (hereinafter referred to as PCs), including notebook PCs.
As one example of the active-matrix type of liquid crystal display devices, a TFT (Thin Film Transistor) type liquid crystal display module is known. This module includes a T19′ type liquid crystal display (TFT-LCD) panel, drain drivers disposed along the long side of the liquid crystal display panel, and gate drivers or an interface unit disposed along the short side of the panel.
Generally, the drain driver internally has a gradation voltage generating circuit, which generates a gradation voltage to be supplied to the pixels of the LCD panel based on a plurality of gradation reference voltages supplied from the interface unit.
Generally, a layer of liquid crystal material is characterized in that, when the same voltage (DC voltage) is applied thereto for an increased length of time, the inclination of such liquid crystal becomes fixed, resulting in occurrence of an after-image or “ghost” phenomenon. This leads to a decrease in the lifetime of the liquid crystal layer. In order to avoid this problem, the liquid crystal display module is arranged so that a voltage to be applied to the liquid crystal layer is converted into an AC voltage periodically; that is, relative to the common voltage to be applied to a common electrode (shared electrode), the gradation voltage to be applied to a pixel electrode is alternately changed in polarity between the positive voltage side and the negative voltage side at constant time intervals.
Drive methodology for applying the AC voltage to the liquid crystal layer includes two known methods: a common symmetry method and a common inversion method. The common inversion method is a method which alternately inverts the common voltage being applied to a common electrode and the gradation voltage being applied to a pixel electrode between positive and negative polarities. The common symmetry method is a method in which the common voltage as applied to a common electrode is kept constant, and the gradation voltage being applied to a pixel electrode is inverted so that it alternately takes positive and negative polarities with reference to the common voltage to be applied to a common electrode.
FIG. 30 is a diagram illustrating the polarity of a gradation voltage (i.e. the gradation voltage to be applied to a pixel electrode) which is outputted from a drain driver to a drain signal line in the case of using a dot inversion method as the liquid crystal display module drive method.
With the dot inversion method, as shown in FIG. 30, at an odd-numbered line in an odd-numbered frame, for example, a gradation voltage (indicated by “•” in FIG. 30), having a negative polarity relative to the common voltage (Vcom) being applied to the common electrode, is applied from a drain driver to an odd-numbered drain signal line; whereas, a gradation voltage (indicated by “∘” in FIG. 30), having a positive polarity relative to the common voltage (Vcom) being applied to the common electrode, is applied to an even-numbered drain signal line. Further, at an even-numbered line in an odd-numbered frame, a positive gradation voltage is applied from the drain driver to an odd-numbered drain signal line, and a negative gradation voltage is applied to an even-numbered drain signal line.
In addition, the polarity per each line is inverted for each frame. More specifically, as shown in FIG. 30, at an odd-numbered line of an even-numbered frame, a positive gradation voltage is applied from the drain driver to an odd-numbered drain signal line, and a negative gradation voltage is applied to an even-numbered drain signal line. Further, at an even-numbered line of the even-numbered frame, a negative gradation voltage is applied from the drain driver to an odd-numbered drain signal line, and a positive gradation voltage is applied to an even-numbered drain signal line.
By use of this dot inversion method, the voltages that are applied to neighboring drain signal lines are opposite in polarity to each other. Thus, it is possible to permit adjacent ones of the currents flowing in common electrodes and/or the gate electrodes of thin-film transistors (TFT) to cancel each other, thereby enabling a reduction of the electrical power consumption.
In addition, the common electrode-flowing current remains lower in level, preventing a voltage drop-down from becoming greater. Thus, the common electrode is stabilized in voltage level, enabling minimization of a decrease in on-screen display quality.
However, currently available PCs with a built-in liquid crystal display module, which employs the above-described dot inversion method, are faced with a problem, as follows. Flickers (flicking noises) can occur on the display screen of a liquid crystal display panel in cases where a specified relationship is present between the timing of AC voltage conversion and an image pattern to be visually displayed (e.g. Windows® exit screen or else), which would result in a decrease in display quality.
This problem is solvable by employing, as the drive method, an N-line (e.g. two-line) inversion method, which inverts the polarity of a gradation voltage being applied from a drain driver to a drain signal line for each N lines (e.g. two lines).
However, in the case of employing such N-line (e.g. 2-line) inversion method as the drive method, there has been a problem, as follows. A pattern of lateral stripes with a pitch equal to N lines appears on the display screen when displaying a single-colored monotone image on the entire display screen, as shown in FIG. 31. This causes a significant decrease in the display quality of the liquid crystal display panel.
The present invention has been made in order to avoid the problems of the prior art as described above, and an object of this invention is to provide a technique that is adaptable for use in a liquid crystal display device and a driving method thereof, which technique serves to avoid unwanted creation of a lateral stripe-like “ghost” pattern on a display screen when inverting the polarity of a gradation voltage for each group of N lines (N≧2), to thereby achieve an increase in the on-screen image display quality.
The above object and new features of the invention will be apparent from the following more detached description of preferred embodiments of the invention, as illustrated in the accompanying drawings.